1. Field of the Invention
The present invention relates to an exposure apparatus of photolithographic equipment for use in the manufacturing of semiconductor devices or the like. More particularly, the present invention relates to an auto focus system of the exposure apparatus that positions a substrate for exposure in photolithographic equipment.
2. Description of the Related Art
Generally, the manufacturing of a semiconductor device includes a photolithographic process in which a fine pattern is formed on a wafer (hereinafter, referred to as a “substrate”). The photolithographic process begins with an exposure process in which a beam of light is directed through a reticle and onto the substrate. In this way, a pattern borne by the reticle is transferred to a layer of photoresist on the substrate. The exposure process is thus crucial in forming a fine pattern on the substrate. In particular, the precision under which the beam of light is focused on the substrate is one very important factor in forming a high-quality pattern, especially a fine multi-layered pattern, on the substrate. To this end, an auto focus system is used to position the substrate in the exposure apparatus such that the image of the pattern borne by the reticle is focused precisely on the plane of the photoresist layer during the exposure process.
A conventional auto focus system irradiates the substrate with a so-called focus beam and detects the beam reflected from the substrate to discern the state of the focus of the exposure apparatus. More specifically, the focus beam is directed onto the substrate obliquely and the focus beam reflected from the surface of the substrate is received by a sensor. The sensor senses the location at which the light is incident thereon to determine the relative position of the substrate. A substrate stage, on which the substrate is supported, is driven based on data produced from the output of the sensor to position the substrate in a focal plane of the exposure apparatus.
Research aimed at improving the resolution of the exposure apparatus is ongoing to meet the demand for more highly integrated semiconductor devices, i.e., devices that have finer circuit patterns. In this respect, it is known that fine patterns can be formed when the exposure light has a relatively small wavelength. Thus, past research has focused on developing and putting into practice light sources that output exposure light having small wavelengths. Typically, a KrF excimer laser emitting light having a wavelength of 248 nm or an ArF excimer laser emitting light having a wavelength of 193 nm is employed as a light source in the exposure apparatus of current photolithographic equipment. Recently, though, an F2 excimer laser has also been employed as a light source.
It is has proven technically difficult to develop light sources that output exposure light having shorter wavelengths than those mentioned above. Thus, an immersion exposure technology, that effectively increases the aperture number of the apparatus, has been suggested as a means for increasing the resolution of the exposure apparatus. The immersion exposure technology employs the existing KrF, ArF or F2 excimer lasers as the light source; however, an immersion medium is interposed between the substrate and an optical system of the exposure apparatus to increase the aperture number. Such immersion exposure technology is disclosed in U.S. Pat. Nos. 6,781,670 and 6,809,794.
More specifically, in exposure apparatus that employs the immersion exposure technology, a liquid (immersion) medium is provided between the substrate and the optical system. However, there is a problem in that the refractive index of the immersion medium varies locally due to bubbles or temperature changes generated therein by the light directed therethrough.
Such local variations in the refractive index of the immersion medium cause a very large problem in the auto focus process. That is, when the focus beam passes through the immersion medium during the auto focus process, the optical path of the beam may assume an unexpected direction if the beam passes through a portion of the immersion medium in which the refractive index varies. In this case, a focus error occurs. This focus error may be also generated by the air or other material through which the focus beam propagates on its way to the substrate. That is, a focus error due to variations in the refractive index of a medium through which the focus beam must pass can be generated in a dry exposure apparatus in addition to an exposure apparatus employing a liquid immersion medium.